Syntax

unsigned long var = val;

var - your long variable name
val - the value you assign to that variable

Example Code

unsigned long time;
void setup()
{
Serial.begin(9600);
}
void loop()
{
Serial.print("Time: ");
time = millis();
//prints time since program started
Serial.println(time);
// wait a second so as not to send massive amounts of data
delay(1000);
}